1. Field of the Invention
The present invention relates to a valve timing regulation apparatus for an internal combustion engine for regulating opening or closing timing of an intake valve or an exhaust valve of an internal combustion engine.
2. Description of Related Art
In a conventional valve timing regulation apparatus for regulating opening or closing timing (valve timing) of an intake valve or an exhaust valve of an internal combustion engine, drive torque is transmitted from a crankshaft as a drive shaft of the engine to a camshaft as a driven shaft through a drive force transmission member. As the drive force transmission member, for example, a ring-like gear or a vane is employed.
The ring-like gear is engaged with a timing pulley and a spline of the camshaft. At least one of those is engaged with a helical spline. The ring-like gear is moved in an axial direction by fluid pressure whereby the camshaft and the timing pulley are relatively rotated to regulate the valve timing of the intake valve or the exhaust valve according to operating conditions of the engine.
Further, in the vane system disclosed in Japanese Patent Laid-Open No. Hei-1-92504, a housing rotated along with the timing pulley houses therein a vane rotated along with the camshaft. The relative rotation phase difference of the vane with respect to the housing is regulated by fluid pressure to thereby relatively rotate the camshaft and the timing pulley so that the valve timing of the exhaust valve is regulated according to the operating conditions of the engine.
However, in the above conventional valve timing regulating apparatus, the camshaft as the driven shaft receives the force on the retard side with respect to the drive shaft by the drive torque applied to the camshaft for opening and closing the exhaust valve. Accordingly, when the fluid pressure does not operate such as when the engine starts, and at the time of low oil pressure such as when it is idling, the opening timing of the exhaust valve is retarded to sometime overlap the opening timing of the exhaust valve and the opening timing of an intake valve. When the opening timings of the exhaust valve and the intake valve overlap, the combustion gases remained in the cylinder of the engine, i.e., internal exhaust gas recirculation (EGR) amount becomes excessively large, and as a result, the startability of the engine becomes deteriorated, and the engine sometimes becomes disabled to start. Further, there is a problem that the unburned fuel is discharged into exhaust gases.
Further, in the case where the phase difference is regulated by fluid pressure, for example, when the number of revolutions of the engine is low and the discharge pressure of a fluid pump is low, the camshaft cannot be moved toward the advance side with respect to the crankshaft, sometimes resulting in disablement of regulation of the valve timing. Also in the case of the low fluid pressure as described, it is contemplated that a pressure receiving area of fluid pressure is increased to render the regulation of the valve timing possible. However, even if the valve timing can be regulated with low fluid pressure, in the case where the discharge amount and discharge pressure of the fluid pump are sufficient with high rotation of the engine, the flow rate of the operating fluid increases and the time required to change the valve timing increases. That is, a problem arises in that the responsiveness lowers. Further, a problem arises in that the size of the apparatus increases. Further, when the operating fluid pressure is lowered due to the trouble of the fluid pump or the like, the valve timing cannot be regulated, and the engine may be stopped.
Still further, it is necessary to advance the camshaft with respect to the crankshaft at the time of start of the engine and at the time of low load. However, when the camshaft is retarded with respect to the optimum valve timing, the period at which both the exhaust valve and the intake valve are opened increases due to the low rotation and the trouble of the exhaust valve. Then, the exhaust gas remains within the combustion chamber so that the necessary amount of air is not taken into the combustion chamber and the exhaust gas is reversed on the intake side. Further there occurs a problem in that the unburned gas is discharged into the exhaust gas.
As a result, the combustion gas, i.e., internal EGR amount remained in the cylinder of the engine becomes excessively large whereby the combustion is unstable, the noxious component amount contained in the exhaust gas increases, and in the extreme case, the engine stops. At the time of start of the engine, startability becomes worsened.